Problem: The scale on a map is 9cm : 5km. If the distance between two cities is 15km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 9cm on the map corresponds to an actual distance of 5km. An actual distance of 15km is the same as 3 $\cdot$ 5km. The distance between the two cities on the map is 3 $\cdot$ 9cm, or 27cm.